Bytecode cache: portable across platforms and deterministic (same source ⇒ same bytes everywhere) - #505
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…form The cache is decoded by Bun executables cross-compiled for a different OS/CPU than the one that produced it, but two things in it depended on the encoder's C++ ABI: - CachedMetadataTable serialized the finalized metadata offset table, whose entries are byte offsets computed from sizeof(Op::Metadata). Those sizes differ between ABIs (everything embedding CallLinkInfo is 16 bytes larger under MSVC), so a table encoded on Linux/macOS and linked on Windows had LLInt indexing past each entry. Serialize per-opcode entry counts instead and lay the table out again on decode via the normal finalize() path. - CachedStringSourceProvider's Bun-only m_sourceLength sat in the base class's tail padding under Itanium but not under MSVC. It didn't validate anything SourceCodeKey doesn't already (the key compares the source hash), so drop it and hand back the provider being decoded against. Also make the encoder's output a pure function of its input so it can be compared across platforms: fixed allocation alignment and page size instead of alignof(max_align_t)/pageSize(), zeroed pages so struct padding is not heap garbage, string switch tables encoded in index order rather than StringImpl-pointer hash order, and UnlinkedHandlerInfo encoded field-wise rather than copied with 30 bits of bit-field slack. The two allocation entry points now static_assert what the compiler can check about a type's portability (alignment, no destructor/array cookie, and for verbatim-copied types: no padding or bit-field slack, no long/wchar_t/long double).
The verbatim-copy check rejected `long` by name to catch the LP64/LLP64 size difference, but int64_t (EncodedJSValue) is `long` under glibc. Only reject `long` where it is the 32-bit outlier.
…agree with Bun embeds this cache in executables it cross-compiles, so the bytes are decoded by a build whose OS, CPU and C++ ABI differ from the encoder's. Two things in the format depended on the encoder: - The metadata table was stored as offsets (lately as offset deltas), which are sums of this build's sizeof(Op::Metadata); anything embedding CallLinkInfo is 16 bytes larger under MSVC, so a table written on Linux/macOS and linked on Windows had LLInt index past every entry. The cache now stores per-opcode entry counts and UnlinkedMetadataTable::layOut() -- the one function that turns counts into offsets, shared with finalize() -- runs on the decoding side (at link() for a persistent payload, as before). - Records are placed in the payload as C++ objects, so any byte the ABI is free to place (tail padding Itanium reuses and MSVC does not, bit-field units MSVC restarts on a type change, holes after a bool/uint8_t/uint16_t member) can land differently. Every type now enters the Encoder through malloc<T> / mallocArray<T> / mallocCopy<T>, which static_assert isPortableRecord<T>(): trivially destructible, alignment <= 8, no long/wchar_t/pointers, and has_unique_object_representations -- i.e. no padding and no bit-field slack, so the layout is fully determined by the declared field sizes. The dozen records that had holes get their fields reordered/widened or an explicit zeroed m_padding; UnlinkedHandlerInfo, VariableEnvironmentEntry, PrivateNameEntry and SymbolTableEntry are encoded through Cached* records instead of copied; CachedStringSourceProvider's Bun-only m_sourceLength is gone (SourceCodeKey already compares hash and length). And the output is made a pure function of the source so that platforms can be compared byte for byte: allocation alignment and encoder page size are constants instead of max_align_t/pageSize(), and hash maps/sets and TDZ environments are written in key order instead of table order (which follows SymbolImpl's per-process hash counter, or a robin-hood table's own address).
…displacement, as the table's last entry does totalSize() of a table backed by persistent entry counts used it as-is and under-allocated the linked buffer by s_offset32TableSize for functions large enough to need 32-bit offsets (ASAN: heap-buffer-overflow writing OpPutToScope::Metadata in CodeBlock::finishCreation).
…tanding in the runtime provider Now a plain uint32_t after a base without tail padding, so both ABIs place it at the same offset (the reason the earlier revision dropped it).
…ings of one source are identical
…a payload cannot wrap it; EncodingOrder: order equal-content keys by symbol kind
… operand order, not argument-evaluation order op_iterator_open, op_iterator_next, op_async_iterator_open and op_instanceof took their two or three value-profile indices as nextValueProfileIndex() calls in one emit() argument list, so which operand got which index was up to the compiler's argument evaluation order: left-to-right under clang for SysV targets, right-to-left under the MSVC ABI. The bytecode was equivalent either way, but not byte-identical across platforms, which the bytecode cache now promises.
Two encodings of the same source could differ: - hash maps/sets and TDZ environments were written in table order, which for SymbolImpl keys follows a per-process hash counter and for robin-hood tables (string switch jump tables) the table's own address; they are now written in key order (EncodingOrder); - UnlinkedHandlerInfo was copied with the 30 unspecified bits next to its 2-bit HandlerType; it is encoded field by field (CachedHandlerInfo); - allocation alignment was alignof(std::max_align_t) and encoder pages were pageSize(), both of which vary by platform and show up as padding; they are constants; - op_iterator_open/op_iterator_next/op_async_iterator_open/op_instanceof took their value-profile indices as several nextValueProfileIndex() calls in one argument list, so which operand got which index followed the compiler's argument evaluation order (reversed under the MSVC ABI); they are sequenced. jsc shell: bytecodeCacheFor(source, kind) returns the payload as a Uint8Array; JSTests/stress/bytecode-cache-deterministic.js encodes a corpus twice and compares.
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WalkthroughJavaScriptCore now produces deterministic, platform-independent bytecode-cache data. Parser cache metadata, metadata tables, symbols, strings, maps, sets, and runtime records use stable representations. New tooling generates cache layouts across build platforms and validates them during releases. ChangesBytecode cache determinism
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Full details: Description checkExplanation The description gives a detailed explanation of the portability and determinism changes, identifies affected implementation areas, describes the new test coverage, and reports verification results. It does not include the Bugzilla link or the exact template headings, but the core required information is substantially complete. Comment |
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I reviewed this PR and didn't find any bugs. Because it touches core bytecode-cache serialization and the bytecode generator, a human look would still be worthwhile.
What was reviewed:
EncodingOrder::comparecovers every key type actually used (unsigned/JSInstructionStream::Offset,RefPtr<UniquedStringImpl>/PackedRefPtr); decode paths rebuild hash tables so encode order is inert, andOffsetLocation::m_indexInTablecarries switch-case order.CachedHandlerInforound-trips all four fields ofUnlinkedHandlerInfovia its(start, end, target, HandlerType)constructor.nextValueProfileIndex()sequencing preserves the existing left-to-right order on non-Windows; fixedencoderMaxAlignment=8is validated by the existingstatic_asserts onCacheEntryalignment.
Extended reasoning...
Overview
This PR makes bytecode-cache output deterministic across processes and platforms. It touches four files: BytecodeGenerator.cpp (sequences four emit() calls so multiple nextValueProfileIndex() side effects have a defined order), CachedTypes.cpp (new EncodingOrder to sort hash-table entries before encoding, new CachedHandlerInfo to encode UnlinkedHandlerInfo field-wise instead of memcpy'ing padding, fixed encoderMaxAlignment/encoderMinPageSize constants, and a rework of CachedCompactTDZEnvironment::encode to sort by contents), jsc.cpp (a shell-only bytecodeCacheFor test helper), and a new stress test.
Security risks
None identified. The changes affect serialization order, alignment padding, and argument-evaluation sequencing — no new attack surface, no untrusted-input parsing changes. The new bytecodeCacheFor function is confined to the jsc test shell.
Level of scrutiny
High. CachedTypes.cpp is the on-disk bytecode-cache format for JSC, and BytecodeGenerator.cpp is on the compile path for all JS. Even though each individual change is small and well-motivated, subtle mistakes here could corrupt cached bytecode or shift value-profile indices.
I verified: all CachedHashMap/CachedInlineMap/CachedHashSet instantiations in the file use key types (unsigned, JSInstructionStream::Offset = unsigned, RefPtr<UniquedStringImpl> variants) that resolve to one of EncodingOrder::compare's overloads; decode rebuilds tables via add(), so encode order does not affect the decoded structure; CachedStringJumpTable values carry m_indexInTable explicitly so sorting the map does not perturb switch dispatch; CachedHandlerInfo matches the UnlinkedHandlerInfo(uint32_t, uint32_t, uint32_t, HandlerType) constructor exactly; CompactTDZEnvironment::Compact is Vector<PackedRefPtr<UniquedStringImpl>> and copyable, and decode still calls sortCompact() so the address-order invariant is restored after load; the value-profile sequencing matches clang's left-to-right order so non-Windows bytecode is unchanged; and the static_asserts guard encoderMaxAlignment=8.
Other factors
The PR includes a targeted stress test and the author reports full JSTests/stress parity via bytecode-cache-test-helper.sh. Nonetheless, given this is core JSC serialization/codegen rather than a mechanical change, I'm deferring so a maintainer familiar with the cache format can confirm — particularly that no consumer relies on the previous (iteration-order) encoding and that lowering max alignment to 8 is safe for every cached record type.
…SourceProviderCache transparent to what gets encoded Records: everything enters the Encoder through malloc<T> / mallocArray<T> / mallocCopy<T>, which static_assert isPortableRecord<T>() -- fixed-width scalars, no padding, no bit-field slack (has_unique_object_representations) -- so every byte in the payload is a named, initialized field and a record's layout follows from its declaration alone. The records that had holes get fields reordered/widened or explicit zeroed m_padding; VariableEnvironmentEntry, PrivateNameEntry and SymbolTableEntry go through Cached* records. Parser (from #433, plus one more case): whether a nested function's body was skipped through the SourceProviderCache changed what the parser recorded for it and for what follows it, and a full collection part-way through generation could drop that cache, so the payload depended on GC timing: - an expression-bodied arrow function's end line/column came from the token after the body when parsed but from the body's last token when skipped; both now use the last token; - NoEvalCacheFeature for a tagged template inside a skipped body was lost; the cache item records and replays it; - after skipping a body whose last token spans lines (a template literal) the lexer resumed on the line that token started on, so every later function's line numbers were short by the lines it spanned (visible in stack traces, not only in the cache); the cache item records where the token ended; - recursivelyGenerateUnlinkedCodeBlock() and the builtin-executable path hold the provider's cache for their duration, and VM::clearSourceProviderCaches() keeps caches that are in use. The stress test also runs under --collectContinuously=1 and --useSourceProviderCache=0.
dylan-conway
changed the title
…le-bytecode-cache
…(verifyLayout() on ARM64 release)
…le-bytecode-cache
…, registry-symbol hashes, atom width; and ** folding through libm
- Parser: sloppy-mode block function hoisting candidates were a HashMap keyed
by FunctionMetadataNode* and iterated to declare the hoisted vars, so their
registers / scope offsets followed arena addresses (ASLR): two runs of
'{ function a(){} function b(){} }' inside any sloppy function encoded
differently. Kept in declaration order now.
- SymbolImpl: registered and private symbols hashed by a process-global
counter, so a VariableEnvironment holding computed-class-field keys (or
builtin @NAMEs) past its inline capacity iterated -- and the generator
assigned scope offsets -- in an order set by how many symbols the process had
made. They hash by contents (as StaticSymbolImpl already does); plain
Symbol() keeps the counter.
- Encoder: a Latin-1 string is written 8-bit even when this thread's atom for
it is 16-bit because an equal 16-bit string was atomized first; and
CachedImmutableButterfly stores indexingMode(), not the cell-lock bits that
share its byte.
- ASTBuilder folds 'a ** b' only on operationMathPow's exact integer-exponent
path; the other cases call the C library's pow(), which rounds differently
across platforms, and left unfolded they get the pow() of where the code runs.
- Parser::restoreLexerState restores the last token's end line too.
Stress-test corpus gains block functions, >9 computed class fields and pow
folds; on main it encodes differently on every run.
…le-bytecode-cache
…verifyLayout() on ARM64 release)
…le-bytecode-cache
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Inline comments:
In `@Source/JavaScriptCore/parser/SourceProviderCacheItem.h`:
- Around line 117-119: Update the declarations of lastTokenEndLine and
lastTokenEndLineStartOffset in SourceProviderCacheItem to use 31-bit bitfields,
matching the surrounding position fields and preserving their existing defaults
and behavior.
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JSTests/stress/bytecode-cache-deterministic.jsSource/JavaScriptCore/bytecompiler/BytecodeGenerator.cppSource/JavaScriptCore/jsc.cppSource/JavaScriptCore/parser/ASTBuilder.hSource/JavaScriptCore/parser/Parser.cppSource/JavaScriptCore/parser/Parser.hSource/JavaScriptCore/parser/SourceProviderCacheItem.hSource/JavaScriptCore/parser/VariableEnvironment.hSource/JavaScriptCore/runtime/CachedTypes.cppSource/JavaScriptCore/runtime/CodeCache.cppSource/JavaScriptCore/runtime/MathCommon.cppSource/JavaScriptCore/runtime/MathCommon.hSource/JavaScriptCore/runtime/VM.cppSource/WTF/wtf/text/SymbolImpl.cppSource/WTF/wtf/text/SymbolImpl.h
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…e:31 like the rest of the struct
…le-bytecode-cache
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I reviewed this PR and the automated bug-hunting pass found no issues. Given the scope — parser state restoration, CachedTypes record layout rework, SymbolImpl hashing changes, and GC/source-provider-cache lifetime — a human review would still be worthwhile.
What was reviewed:
CachedTypes.cppfield reordering /m_paddingadditions and theisPortableRecord<T>()gate on all encoder allocations.SymbolImpl::hashForSymbol— registered/private symbols now content-hashed; plainSymbol()still uses the counter, so identity-hash uniqueness is preserved.- Parser
m_lastTokenEndPositionthreading andSourceProviderCacheItemreplay for multi-line last tokens and tagged-template feature bits. makePowNodefolding restriction and the sequencednextValueProfileIndex()calls — both preserve prior Itanium-ABI behavior.
Extended reasoning...
Overview
This PR touches 15 files across the parser (Parser.cpp/h, SourceProviderCacheItem.h, ASTBuilder.h), bytecode generator, the bytecode-cache encoder (CachedTypes.cpp, ~400 lines changed), runtime (CodeCache.cpp, VM.cpp, MathCommon.*), and WTF (SymbolImpl.*). The goal is byte-identical bytecode-cache output for identical source, independent of process state, GC timing, host pageSize()/max_align_t, hash-table iteration order, and compiler argument-evaluation order.
Security risks
None identified. The changes are to serialization layout, parser bookkeeping, and hash derivation. No new external inputs, auth, or privilege boundaries. The new bytecodeCacheFor() is a jsc-shell-only test helper.
Level of scrutiny
High. This is engine-core work with several subtle correctness dimensions:
- Record layout rework in
CachedTypes.cpp: many structs have fields reordered or widened, bit-fields replaced with explicit bytes, and a newhas_unique_object_representationsstatic-assert gate. A missed decode-side mirror or alignment mismatch would corrupt loaded bytecode. - Parser semantics:
parseFunctionBodynow usesm_lastTokenLocationinstead of the current token for expression-body arrow end position, and the cache-skip path resets the lexer fromm_endPositioninstead ofm_startPosition. These change observable line/column numbers and lexer resume state. SymbolImplhashing: registered and private symbols move from a per-process counter to a content-derived hash. This is safe because such symbols are unique-per-registry / unique-per-VM by contents, but it's a foundational WTF change.- GC interaction:
VM::clearSourceProviderCaches()now skips caches with refcount > 1, andrecursivelyGenerateUnlinkedCodeBlockholds a ref for its duration. - Annex B hoisting now iterates a source-order
Vectorinstead of aHashMap, changing declaration order inm_declaredVariables.
Other factors
The PR includes a stress test exercising the corpus under three configurations, and the description reports full JSTests/stress parity with main plus cross-platform byte-identity verification. The one CodeRabbit nit (bitfield layout) was addressed. No bugs were found by the automated pass, but the breadth and depth of engine-internal changes here place it well outside what should be auto-approved.
…der whose CPU reserves a different number treats the payload as foreign BytecodeGenerator numbers a code block's locals after the LLInt/baseline callee-save area, whose size comes from the encoding CPU's register set. It is 4 registers on x86-64 and arm64 alike, which is what makes payloads interchangeable between them; this makes that dependency explicit instead of coincidental.
…into claude/bytecode-cache-deterministic
…l Latin-1 (no copy of genuine 16-bit strings)
… their bytes Vector<std::pair<const UniquedStringImpl*, uint16_t>>::operator== compares with memcmp (both halves are 'simple' types), and each pair carries six bytes of padding whose contents depend on the allocator and on how the compiler stores the pair. Two identical environments then compared unequal on some machines and equal on others, so whether a nested function's parent private-name environment was shared with the previous identical one -- and with it the payload's size -- varied from machine to machine for the same source. Compare with std::ranges::equal (pair::operator==).
…ly on every platform A payload encoded on one platform is decoded in place on another (bun build --compile cross-compiles), so every record CachedTypes.cpp writes must have the same size, alignment and member offsets under the Itanium and Microsoft C++ ABIs and on every architecture. Nothing enforced that; it happens to hold. Tools/Scripts/dump-bytecode-cache-layout.py recompiles the translation unit containing runtime/CachedTypes.cpp with -fdump-record-layouts and prints, in a target-independent spelling, the layout of every record that ends up in a payload: everything deriving from CachedObject<> / VariableLengthObjectBase, the cache entry headers, and the value types those hold or store in their tails (following members, bases and Cached containers' element types, but not CachedObject<T>'s T, the in-memory source type). The linux, musl, macos and windows images write it next to the libraries as bytecode-cache-layout.txt (~3 s), and the release job diffs every platform's copy against linux-amd64 before publishing.
…ode --experimental-strip-types elsewhere) instead of Python
dylan-conway
changed the title
…(InlineMap is not sorted; its order is already deterministic); registered symbols hash purely by contents; Annex B candidates are a plain Vector; EncodingOrder is a bool less + std::sort; private-name environments hash with computeHash; SourceProviderCacheItem's new fields appended instead of reordering; shorter comments; <ranges> with the other system includes; static_assert against encoderMaxAlignment; stress test plans named; layout check fails on a missing dump and covers the freebsd and android images too
…VariableCandidates') so m_lexicalVariables stays at the cache line on ARM64
… parser's synthetic names); Symbol.for() keeps the counter
…string (already computed by the lookup) instead of rehashing; SymbolImpl's constructors are as upstream again
… (MathCommon untouched)
…tream; the cross-platform comparison simply avoids literal non-integer exponents (ParserModes.h, ASTBuilder.h, UnlinkedFunctionExecutable.cpp, UnlinkedCodeBlock.cpp back to upstream)
…pstream): with the parse/skip divergences fixed, dropping caches on a full GC no longer changes the output -- verified under --collectContinuously and a 32 KB max heap on the corpus and typescript.js
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Inline comments:
In `@Dockerfile.musl`:
- Line 104: Pin the nodejs dependency in the Dockerfile.musl APK installation
used by dump-bytecode-cache-layout.ts, either to the tested package revision or
through an Alpine repository snapshot, while preserving the existing package
installation and runtime behavior.
In `@Source/JavaScriptCore/parser/Parser.cpp`:
- Around line 2392-2394: Update arrow-function expression-body metadata to use
m_lastTokenEndPosition.column() for endColumn and its line for
functionInfo.endLine, while retaining m_lastTokenLocation for token-start
fields. Add cache-enabled and cache-disabled coverage for simple and multiline
final tokens.
In `@Tools/Scripts/dump-bytecode-cache-layout.ts`:
- Around line 18-21: Update splitCommand() to parse quoted command arguments
without stripping Windows backslashes, preserving paths such as
C:\work\clang-cl.exe before compileCommand() passes them to recordLayouts() and
execFileSync(). Retain support for quoted and unquoted arguments while applying
platform-correct escaping rules.
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…esting m_steps for null (a value-profile-only table has zero steps)
…arguments (clang-cl lays out more records in this TU; pointer-like templates' arguments dragged GC types into the Windows dump)
…wed by the local type)
…ds every tarball); the per-image dumps stay
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…table JSC bytecode cache) (#40270) ## What `bun build --compile --bytecode --target=<other OS>` produced executables that segfault on launch (#18416): the embedded JSC bytecode cache held a per-function metadata offset table computed from the *building* machine's `sizeof(Op::Metadata)`, and the target decoded it with its own. oven-sh/WebKit#505 and oven-sh/WebKit#521 (both merged) make the cache format independent of the encoding platform — metadata stored as entry counts and laid out by the decoder; every serialized record compile-time-asserted to be padding-free with a fixed size; encoder output a pure function of the source (hash-table order, address order, host page size, host FPU NaN, SourceProviderCache re-parses all removed as inputs). This PR bumps WebKit to that build, adds the test that keeps it true, and fixes the Bun-side places that fed the encoder something non-deterministic. ## Changes - **WebKit** → `7259739917cd`. - **`node:vm` cachedData** (`NodeVM.cpp`, `NodeVMScript.cpp`, `NodeVMSourceTextModule.cpp`): `produceCachedData` / `createCachedData()` now generate the complete code-block tree (`recursivelyGenerateUnlinkedCodeBlockFor{Program,ModuleProgram}`) instead of serializing whatever the VM's CodeCache happened to hold, so the data contains every nested function and no longer depends on what ran before or on an attached debugger; module cached data is produced and looked up with the key JSC uses for module code (strict, `ModuleType`), so `SourceTextModule` cached data round-trips. This makes `cachedData` larger (all functions instead of the top level only) and a little slower to produce. - **Standalone executables**: one new `Flags::CROSS_COMPILED_BYTECODE` bit, set when `--bytecode` output goes into an executable for a different os/arch/libc than the bun that built it; at startup it raises the `cross_compiled_bytecode` crash-report feature so such crashes are identifiable. - **Builtin private names** (`BunBuiltinNames.cpp`): Bun's `@name` symbols are now static `SymbolImpl`s with content-derived hashes, like JavaScriptCore's own, instead of per-VM `PrivateSymbolImpl`s hashed by the process-wide symbol counter — so the bytecode generated for a builtin module no longer depends on how many `Symbol`s the process created before (one was enough to reorder `node:http2`'s constants). Also ~600 fewer allocations per VM; startup time and RSS unchanged within noise. - **Exact-length bytecode embed**: `--compile` previously counted up to 128 never-written buffer bytes after each module's bytecode in its length; the executable now contains exactly the payload, so two builds of the same input are byte-identical. - `bun:internal-for-testing`: `internalModuleBytecode()` exposes the builtin-module bytecode + external string table `--compile --bytecode` embeds, for the test below. ## Test `test/bundler/bundler_bytecode_portable.test.ts` encodes a corpus that reaches every kind of object the cache serializes and 170/194 opcodes — `features.js`, `records.js` (one section per record type), a generated `big.js` (32-bit metadata tables, wide operands, multi-page payloads), `shapes.js`, `source-forms.js` (hashbang, CRLF, U+2028/9, `\u{}` escapes, legacy octal — via `vm.Script`), `module.js` / `esm/` (module code), a builtin-syntax module, 7 individual libraries and a 64-library bundle from `test/node_modules`, and `all.js` (everything in one payload) — through `bun build --bytecode` (± `--minify`), `vm.Script`, `vm.SourceTextModule` and the builtin encoder, and compares one inline snapshot of the SHA-256s on every CI platform. Because the encoder's output depends only on the source, equal hashes on Linux/macOS/Windows × x64/arm64 (+ musl) mean every platform writes — and therefore reads — the same bytes. If the snapshot ever fails on only *some* platforms, that is a portability regression in JSC, not a snapshot to update (it caught one during this PR: x86 and ARM folding `0/0` to differently-signed NaNs, fixed in oven-sh/WebKit#521). The same file checks that the output is independent of the encoding process (GC timing, SourceProviderCache, a busy VM, internal modules), that every corpus build loads from its cache with identical output, that `--compile --bytecode` executables (CJS and ESM, including a Worker entry) run from the embedded bytecode, and that an unusable `.jsc` is a cache miss rather than a crash. Also: `vm.test.ts` (SourceTextModule accepts its own cached data), `capture-stack-trace.test.js` (line numbers after a multi-line template literal, with and without the SourceProviderCache — one of the parser fixes in oven-sh/WebKit#505), `bun-build-compile.test.ts` (`cross_compiled_bytecode` set only for a foreign target). Fixes #18416 --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
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Makes a bytecode cache payload (what
generateProgramBytecode/generateModuleBytecode/encodeBuiltinFunctionwrite) portable and reproducible: a payload encoded on one platform decodes on the others (sobun build --compile --bytecodecan cross-compile), and no byte in it depends on the process, the compiler's layout choices, or GC timing, so the same source gives the same bytes everywhere and payloads can be compared across machines. Supersedes #389 and the determinism half of #433.Portability (was #389)
sizeof(Op::Metadata); those structs are laid out differently under the MSVC ABI, so a Linux-built payload walked the table at the wrong offsets on Windows and crashed. It now stores per-opcode entry counts (CachedMetadataSteps), and the decoder lays the table out with its own sizes viaUnlinkedMetadataTable::expandSteps.GenericCacheEntryrecordsCodeBlock::llintBaselineCalleeSaveSpaceAsVirtualRegisters()andisUpToDate()rejects a payload built with a different value, instead of running with mis-sized frames.Cached*record must have the same size/alignment/offsets under the Itanium and Microsoft C++ ABIs. That holds today but nothing enforced it:Tools/Scripts/dump-bytecode-cache-layout.tsdumps them from each platform's build (linux, musl, macos, windows images; ~3 s) and the release workflow diffs every platform against linux-amd64 before publishing.Determinism
What reached the bytes before, and what replaces it:
Encoder / records (
CachedTypes.cpp)CachedHashMap/CachedInlineMap/CachedHashSet/CachedCompactTDZEnvironmentwrote entries in iteration order — forSymbolImplkeys a per-process hash counter, for robin-hood tables (stringswitchjump tables) the table's own address, for TDZ environmentsStringImpl*order. Now written in key order (EncodingOrder: contents, then symbol kind). Decode is unaffected.UnlinkedHandlerInfowas memcpy'd with 30 unspecified bits beside its 2-bitHandlerType; it goes through aCachedHandlerInforecord of fouruint32_ts. (Padding inside the other records is the page's zeros.)Vector<std::pair<UniquedStringImpl*, uint16_t>>::operator==, which memcmps the pairs' six padding bytes, so whether two environments matched (and the payload's size) varied by machine. Compared pair by pair, hashed by value.alignof(std::max_align_t)(16 on x86-64 Linux/macOS, 8 elsewhere), encoder pages werepageSize(); both fixed (8 / 4 KB).Bytecode generator / parser output
op_iterator_open,op_iterator_next,op_async_iterator_open,op_instanceoftook their value-profile indices as severalnextValueProfileIndex()calls in oneemit()argument list; clang under the MSVC ABI evaluates right-to-left, so Windows numbered them backwards. Sequenced (matching the existing non-Windows order).HashMapkeyed byFunctionMetadataNode*and iterated it to declare the hoisted vars, so their registers / scope offsets followed arena addresses:function f() { { function a(){} function b(){} } }encoded differently on every run (ASLR). Kept in declaration order.VariableEnvironmentwith computed-class-field keys past its inline capacity iterated — and got scope offsets — in an order set by how many symbols the process had created. They take the contents hash the registry lookup already computed (asStaticSymbolImpldoes);Symbol()andSymbol.for()keep the counter.CachedImmutableButterflystoresindexingMode(), not the cell-lock bits sharing that byte.Parser /
SourceProviderCache(from #433, plus one further case). Whether a nested function's body was skipped through the cache changed what got recorded (and the cache is dropped on every full GC, so which path a function took depended on GC timing):NoEvalCacheFeaturefor a tagged template inside a skipped body was lost — recorded in the cache item and replayed;x => \a\nb\nc`; function after() { new Error().stack }reportsaftertwo lines early onmain(correct with--useSourceProviderCache=0`). The cache item now records where the token ended;Test.
jscgetsbytecodeCacheFor(source, "program"|"module") → Uint8Array;JSTests/stress/bytecode-cache-deterministic.jsencodes a corpus (string switches, classes with private/computed members incl. >9 computed fields, generators, try/finally, TDZ, sloppy block functions, multi-line arrow bodies, tagged templates) twice and requires identical bytes, under the default configuration,--collectContinuously=1and--useSourceProviderCache=0.Verified: typescript.js (8.8 MB → 13.6 MB payload) and the corpus encode byte-identically across processes, across cache on/off, and under
collectContinuously(onmain: ~7 000 differing bytes per process for typescript.js, and the stress-test corpus encodes differently on every run); identical between Linux x64 and Windows x64 builds. FullJSTests/stressplain and throughbytecode-cache-test-helper.sh: same results asmain.